Is it oil canning or is it a low spot? Oil canning would indicate an excess of metal (stretched area) which is highly unlikely seeing you welded there. It is more likely a low spot cause by the metal shrinking as you welded (mig I'm assuming) Do as Brad says if it's oil canning, if it's a low spot you can try planishing the haz (heat affected zone) with a hammer and dolly to stretch it and bring the low spot up.
Thats a low spot from welding. Get a dolly behind the weld and hammer on dolly in the area of the low spot. It will start to bring it up. Mig welds are hard though so it may take some good hard whacks.
Can you get to the back side? Us older guys will raise the low spot up and then use a small tip on torch have a wet clean rag heat spot untill red work with a hammer then cool with wet rag. Work fast its sort of an art.
Before I suggest to someone how to fix an oil can, it is best to find out what kind of oil can you are dealing with, first and foremost.
I consider there are two kinds, a tight oil can and a loose oil can, each will require a different method of repair.
Tight oil can
This is almost exclusively caused by body damage, whether a dent, glancing crease, or media blast damage, and is especially noted by displaced metal that will oil can when considerable pressure is applied, and may or may not forcibly spring back. When the body damage occurs, it stretches the panel throughout the dent or crease. A typical dent, whether straight in or a glancing blow, will have direct and indirect damage. The direct damage goes inward, stretching the panel as it goes. The indirect damage, is a much lesser amount of spring-back, compounded by the internal stretch pushing outward circumferentially, and you will see an outward bulge around the perimeter of the dent/damage. Although the initial inclination may be to shrink this outward bulge, for the most part this adjacent area is relatively damage free, it is mainly being spread outward by the stretch forcing outward.
Shrinking the center damage will start the process of relieving the stresses pushing outward, relaxing some of the bulge surrounding the dent.
After a bit of shrinking, using the shot bag against the outside of the crease/dent and some light taps with a flat body hammer or slapper from the inside will help to start manipulating the crease/dent back into it's original place. I would add that too much shrinking all at once may give you the loose oil can, so profile templates are highly recommended as they work well to let you see how the panel is reacting so you don't go too far too quick.
Tight oil can, part two
Where some tight oil cans from dents may be challenging to determine where to start your shrinking (if it doesn't have an obvious sharp crease to show where to work from) the following process will normally find the area that needs shrinking.... Cycle the oil can in and out a couple times in order to find the outer perimeter. If it helps to mark it with some painters tape, a sharpie, so be it, use whatever works. Now using your thumb from one hand apply slight pressure on a point on this perimeter. Use the other hand to cycle the oil can again, using the same pressure as before. Keep moving your pressure point around the perimeter and cycle the oil can for each spot until you get to a point on the perimeter where the pressure will keep the oil can from cycling, it locks it from moving. This should identify your sweet spot that needs shrinking, and be aware that there may be more than one sweet spot needing attention.
Loose oil can
This is typically caused by welding, over-eager torch shrinking, or shrinking something when you should have stretched, (or fatigue over the many years that has caused a larger panel/hood to settle). Any panel will shrink from heat, causing the crown to draw in from the surrounding area. This is especially noted by a loose, easily flopped back and forth oil can. This is fixed by stretching, typically in the area of the weld and HAZ.
Loose oil can part two
In some cases we'll see that a dent (or tight oil can) has actually caused a loose oil can in the outer reaches in the adjacent area. The direct force
(dent) may have caused displacement of the inherent stresses of the panel (crown) such that it pulled at the adjacent metal elsewhere, resulting in a loose oil can outside the area of the dent. Here the loose oil can should be left alone and focus on removing the stretched area (dent) that moved the panel. Once the dent is removed, this action alone should correct the loose oil can in the adjacent area.
Thanks for the detailed reply. The area I'm most concerned with bulges out past the natural contour of the panel and requires significant pressure to pop in. So the metal is stretched, but not due to being hit. I would say this falls into your "Tight oil can, part two" So what would be the best method to repair this?
I'd suggest that you are misreading your panel, the circled area is not stretched at all.
Take a piece of paper and lay in front of you on table/desk, the outer perimeter will represent a weld line. Hold one corner and push the opposite corner on the long edge of the paper together. You start to see a pucker in the middle. do the same for the opposite corner along the short edge of the paper. You see a pucker here as well. This represents what happens to your circled area as the weld cools and shrinks in length. In no circumstance did any of the paper stretch in the middle, and by the same token neither did your circled area. You have two perpendicular welds along the outer perimeter of the bulge that shrinks during the weld cooling and makes it APPEAR to have an excess of metal in the middle. The welds and HAZ need to be planished (stretched) to relieve this pressure being forced inward. If the welds have caved inward, you may find it easier to hammer from the inside with flat dolly on the outside in order to persuade the weld outward as you planish. This repair does not require any shrinking at this point.
I would add that if there was no damage to the panel before your patching efforts, and the only thing you did was weld, then shrinking is the only thing that has taken place. You will never repair a shrinking issue with more shrinking.
OK, sorry for the confusion, but I reviewed the panel again and see the following.
The blue circle is bulging out and takes a lot of force to pop in and moderate pressure to pop back out. I think the blue circle is bulging out more than it should to match the contour of the larger general area.
The red circle is in and takes moderate pressure to pop out from the inside, but it will automatically pop back in after releasing pressure from the inside. Additionally, when pushing it (red circle) out from the inside, the green circle will pop in and stay in as long as the red circle area is being pushed out from the inside.
Maybe stretch the red circle first and then shrink the blue?
Any stretching at all should be on the weld and heat affected zone ONLY. Leave the in/out bulges alone until you address the weld. Doesn't matter if it is bulging inward or outward, it is metal attempting to go somewhere as a shrinking weld seam is pulling it together. You have a weld that is shrinking, metal about 6+ inches away is far enough outside the heat that it remains unaffected, and what's trapped in the middle is forming puckers from the differing forces. If the weld is long enough, and shrinks enough, you will indeed have metal pushed both upward and downward as it tries to go somewhere, forming a sinusoidal wave.
Ask it ten different ways, the weld is still the cause, and it won't change where the repair should start. You start stretching any of that circled sheet metal and you're adding to your problem.
Notice all your circles of bulging metal are right next to a weld seam that has shrunk... Guess what caused this mess? You haven't touched anything inside those circles, don't start now. I think you'll see after a few planishing rounds on the welds that the bulges will lose some of their "stiffness". As you stretch out that shrunken weld it will have less pull on those areas and the puckers will loosen.
Well put Robert. Ksungela, you are looking at the result (oil can, high spot low spot etc), and thinking it is the cause. The cause is the HAZ (heat affected zone) The oil can, low spot, high spot whatever) is the result. Stretch the weld area and the affected areas will improve. If it's a MIG weld it'll help to grind down to flush, or near flush before you start stretching. Use an appropiate dolly for the curvature of the panel, get behind the weld and hammeron the dolly. Meaning the dolly is directly underneath your hammer blows. If you are doing it right you will hear a ringing sound each time you strike the metal. That sound means you are stretching. Mig welds are hard so it will take some effort to planish it out.
Just to add to what Robert is saying, he is telling you to stretch the weld and HAZ area. He means for you to do this by planishing the area. Hammer on dolly method to displace the metal xausing it to "grow" in area. this will release the tension in your circled areas.
Sorry Chris, I had started this before you posted and got side tracked. I did not mean to cover the same ground you did again.........
Well, I took the bed off today and had access to the weld area. I hammered the weld area from top to bottom and what do you guess happened? The oil canning went away and the panel is pretty close to its original curvature. Not perfect, but I'm comfortable moving on with the body work from this point Thanks for all your help.